In the natural state, salmonid eggs are deposited in gravel and suffer substantial mortalities from the time of egg deposition to the fry emergence stage, which occurs some months later.
Under normal conditions, about 80 percent of all eggs carried by a female perish. There are a number of reasons for this. Firstly, not all of the eggs are fertilized when they are deposited and not all of them are successfully buried. Also, fungus from dead eggs tends to spread to healthy eggs. In addition, some of the eggs are eaten by predators at the time of deposition, and some are covered with silt and suffocated, whereas others dry out as a result of a drop in water level.
Consequently, attempts have been made in the past to provide an incubator which will improve the survival rate of the salmonid eggs.